Tunica intima: endothelium and basement membrane, layer of elastic fibers (internal elastic lamina, made up of elastin) Tunic media: smooth muscle, external elastic lamina. Tunica externa: elastic and collagen fibers. Elastic arteries: largest diameter (aorta, pulmonary trunk, high density of elastic fibers, act as a pressure reservoir, stretches as ventricle contracts. Muscular arteries: most arteries, thick tunica media. Resistance vessels: diameter determines blood flow to specific tissues, resistance is proportional to 1/d4. Site of exchange of o2, co2, nutrients, wastes, and hormones between the blood and interstitial fluid. Only basement membrane and endothelium for easy diffusion. A capillary bed is a network of capillaries (10 100) The last branch of an arteriole before the capillary bed is the terminal arteriole. Blood only flows through 25% of capillaries at a given time. Veins: thinner walls with less smooth muscle than arteries.